Enemy ships now have a chance to drop 'energy' when destroyed. This energy can be used to purchase the four different powerups which previously would  spawn into the world as a pickup.

Once enough energy is collected (and any other pre-requisites are met) the powerup name and corresponding number will light up. Pressing the number (1, 2,3 ,or 4) or using Q/E (keyboard) or L/R (controller will navigate/select items in the powerup menu. Once selected, if the powerup is available, press Space (keyboard) or A (controller) to purchase it.

I personally like going for high attack speed right away, then slowly building up the 4 flank ships. 

I hope anyone who tries out this game will enjoy trying to survive as long as possible!